1. Field of the Invention
The present invention relates to belts for a continuously variable transmission. The belts are formed by supporting a large number of metal elements on a metal ring assembly wherein multiple sheets of endless metal rings are layered.
2. Description of the Prior Art
When a metal belt of a continuously variable transmission is gripped by V-faces of its drive pulley and driven pulley, tension is generated in the metal belt. Since the tension varies depending on the drive power and the braking power produced by the two pulleys, the tensile stress applied to each metal ring changes periodically with the circulation of the metal belt. Moreover, since the circulating metal belt is bent where it wraps around the pulleys and is stretched in the chord parts between the pulleys, the flexural stress applied to each metal ring also varies periodically. As a result, the total stress (the sum of the above-mentioned tensile stress and the above-mentioned flexural stress) applied to each metal ring varies periodically during one revolution of the metal belt.
Japanese Patent Application Laid-Open No. 57-57938 has therefore proposed that by applying a) a residual compressive stress to the radially outer surface of each metal ring of the metal ring assembly and b) a residual stretching stress to the radially inner surface, the median of the stress amplitude (stress median) applied to each metal ring, which varies periodically, is made as close to 0 as possible thereby attempting to extend the wear life of the metal belt.
The above-mentioned conventional arrangement does not differentiate between the metal ring of the innermost layer from the other metal rings. A residual compressive stress is applied to the radially outer surfaces of all the metal rings, and a residual stretching stress is applied to the radially inner surfaces. Therefore, the durability of the entire metal belt is restricted by the durability of the metal ring of the innermost layer which is used under severe conditions such as those resulting from large changes in tension. Moreover, the processing needed in order to apply the residual compressive stress and the residual stretching stress to the radially outer and inner surfaces of each metal ring becomes a principal cause for increased cost.